Auckland’s brand new fleet of $400 million electric trains are barred from running at top speed, but there is debate over what is behind the go-slow.

Auckland Transport spokesman Mark Hannan said the electric trains were restricted from travelling at up to 110 kilometres per hour in order to fit in with timetables designed around the 39 slower diesel passenger trains still in operation, which top-out at about 80kmh.

But the Rail and Maritime Transport Union (RMTU) said the 57 brand new electric trains would remain as slow as the diesel fleet until a costly upgrade to onboard software is made.

Hannah said when the diesel passenger trains were removed from the network the electric trains would reach their full potential.

At this stage the only benefits commuters will be noticing from the electric trains is improved comfort and noise levels, Hannan said.

‘‘We can’t really get the proper benefit from them until the full roll out when everything is electric which will be the middle of next year.’’

Also new timetables would need to be introduced and software which controls the trains’ speed, called European Train Control Systems (ETCS), would need to be reprogrammed to improve transit times, Hannah said.

The ETCS is a train protection system designed to help train drivers and ensure they stick to speed limits and obey signal rules to prevent collisions.

The system provides a speed range the driver can operate the train before it intervenes to limit speed.